The present invention generally relates to the diagnosis of sleep disorders and more particularly pertains to the monitoring of a patient's breathing while asleep. Once the cause of sleep disorder has been identified, an appropriate treatment can be prescribed.
Sleep problems may have any of a number of causes including a variety of breathing disorders. For example, obstructive sleep apnea syndrome (OSAS) is a well recognized disorder which may affect as much as 1-5% of the adult population. OSAS is one of the most common causes of excessive daytime somnolence. OSAS is most frequent in obese males, and it is the single most frequent reason for referral to sleep disorder clinics.
OSAS is associated with all conditions in which there is anatomic or functional narrowing of the patient's upper airway, and is characterized by an intermittent obstruction of the upper airway occurring during sleep. The obstruction results in a spectrum of respiratory disturbances ranging from the total absence of airflow (apnea) to significant obstruction with or without reduced airflow (hypopnea and snoring), despite continued respiratory efforts. The morbidity of the syndrome arises from hypoxemia, hypercapnia, bradycardia and sleep disruption associated with the apneas and arousals from sleep.
The patholophysiology of OSAS is not fully worked out. However, it is now well recognized that obstruction of the upper airway during sleep is in part due to the collapsible behavior of the supraglottic segment during the negative intraluminal pressure generated by the inspiratory effort. Thus, the human upper airway during sleep behaves as a Starling resistor, which is defined by the property that the flow is limited to a fixed value irrespective of the driving (inspiratory) pressure. Partial or complete airway collapse can then occur associated with the loss of airway tone which is characteristic of the onset of sleep and may be exaggerated in OSAS. Central sleep apnea is associated with the failure of the body to automatically generate the neuro-muscular stimulation necessary to initiate air control a respiratory cycle at the proper time. Central sleep hypopnea has a similar cause and work associated with the use of electrical stimulation to combat the disorder is ongoing.
Detection of respiration is at the heart of physiological monitoring done during sleep. It is essential to establish that a subject is normal or to identify pauses in breathing and obstructive episodes in which abnormal airflow accounts for the arousals seen in patients with obstructive apnea, upper airway resistance syndrome, and sever snoring. The current standard form of monitoring of breathing is the thermistor or thermocouple, a temperature sensitive device placed near the nose and mouth which detects a change in temperature (hot air during exhalation, and cooler ambient air during inhalation). However, the thermistor and thermocouple devices, which lack the ability to accurately quantitate airflow once detected as they are nearly "all or none devices". Nonetheless, such devices are currently used in attempt to roughly quantify temperature changes. Other devices have been used to directly monitor airflow (pneumotachograph), but all rely on the direct capture and measurement of the volume of air passing through the nose or mouth and thus require a mask to capture the air.
More recently, it has been proposed that the contour of the airflow signal from the true flow signal (as opposed to an indirect monitor-like temperature) can be used to indicate states of abnormal upper airway patency resulting in reduced airflow (hypopnea) or elevated airway resistance with no reduction in airflow in addition to the presence of complete cessation of breathing (apnea). A technique previously proposed requires the combination of a nasal cannula with a sensitive pressure transducer. When used this way, as opposed to as a device to deliver oxygen, the interaction of the cannula tip with the human nostril creates a form of pneumotachograph head, such that the drop in pressure from inside the nose to outside (sensed by the nasal cannula) is proportional to the airflow. This signal is remarkably proportional to the signal from a calibrated pneumotachograph attached to true mask, and thus provides a very comfortable, sensitive way to monitor breathing during sleep. However, two limitations are inherent in the use of such device namely, its inability to detect mouth breathing and its poor sensitivity to very small breaths.
A more accurate approach is therefor needed that provides both a quantitative measure of airflow as well as the ability to detect mouth breathing and small breaths. Additionally, it is most desirable to provide a device with such capability that is relatively inexpensive, easy to use, and minimizes any discomfort to the patient.